1. Field of the Invention
Electrical storage systems have long been state of the art and include in particular batteries, but also so-called supercapacitors, short supercaps. In particular so-called lithium-ion batteries are being discussed in the field of novel applications such as electromobility, because of the high energy densities that can be realized with them, but they have already been used for a number of years in portable devices such as smartphones or laptop computers. These conventional rechargeable lithium-ion batteries are in particular distinguished by the use of organic solvent-based liquid electrolytes. However, the latter are inflammable and lead to safety concerns regarding the use of the cited lithium-ion batteries. One way of avoiding organic electrolytes is to use solid electrolytes. However, such a solid electrolyte has a conductivity that is usually clearly smaller, i.e. by several orders of magnitude, than that of a corresponding liquid electrolyte. In order to obtain acceptable conductivities and to be able to utilize the advantages of a rechargeable lithium-ion battery, such solid-state batteries are nowadays especially produced in the form of so-called thin film batteries (TFB) or thin film storage elements which find their use in particular in mobile applications, for example in smart cards, in medical technology and sensor technology as well as in smartphones and other applications which require smart, miniaturized and possibly even flexible power sources.
2. Description of Related Art
An exemplary lithium-based thin film storage element has been described in US 2008/0001577 and basically consists of a substrate on which the electronically conductive collectors for the two electrodes are deposited in a first coating step. In the further manufacturing process, the cathode material is first deposited on the cathode collector, usually lithium cobalt oxide, LCO. In the next step, a solid electrolyte is deposited, which is usually an amorphous material including the substances lithium, oxygen, nitrogen, and phosphorus, and which is referred to as LiPON. In the next step, an anode material is deposited so as to be in contact with the substrate, the anode collector, and the solid electrolyte. In particular metallic lithium is used as the anode material. When the two collectors are connected in electrically conductive manner, lithium ions will migrate through the solid-state ion conductor from the anode to the cathode in the charged state, resulting in a current flow from the cathode to the anode through the electrically conductive connection of the two collectors. Vice versa, in the non-charged state migration of the ions from the cathode to the anode can be enforced by applying an external voltage, whereby the battery is charged.
A further thin film storage element is described in US 2001/0032666 A1, by way of example, and also comprises a substrate onto which different functional layers are deposited.
The layers deposited for such a thin film storage element usually have a thickness of about 20 μm or less, typically less than 10 μm or even less than 5 μm; a total thickness of the layer structure can be assumed to be 100 μm or less.
In the context of the present application, thin film storage elements refer to rechargeable lithium-based thin film storage elements and supercaps, by way of example; however the invention is not limited to these systems but may as well be used in other thin film storage elements, e.g. rechargeable and/or printed thin film cells.
A thin film storage element is generally manufactured using complex coating processes also including patterned deposition of the individual materials. Very complicated patterning of the exact thin film storage elements is possible, as can be seen from U.S. Pat. No. 7,494,742 B2, for example. In case of lithium-based thin film storage elements, particular difficulties are moreover encountered due to the use of metallic lithium as an anode material because of the high reactivity thereof. For example, metallic lithium has to be handled under preferably water-free conditions since otherwise it would react to form lithium hydroxide and the functionality as an anode would no longer be ensured. Accordingly, a lithium-based thin film storage element must also be protected against moisture by an encapsulation.
U.S. Pat. No. 7,494,742 B2 describes such an encapsulation for the protection of non-stable constituents of a thin film storage element, such as, e.g., lithium or certain lithium compounds. The encapsulation function is here provided by a coating or a system of different coatings which may fulfill further functions as part of the overall design of the battery.
In addition, as described for example in document US 2010/0104942, under the manufacturing conditions of a lithium-based thin film storage element, in particular during annealing or heat treatment steps which are necessary for the formation of crystal structures suitable for lithium intercalation, undesirable side reactions of the mobile lithium ions with the substrate will occur, since the lithium has a high mobility and can easily diffuse into common substrate materials.
A further issue with thin film storage elements relates to the substrate materials employed. The prior art describes a multiplicity of different substrate materials, such as, for example, silicon, mica, various metals, and ceramic materials. The use of glass is also often mentioned, but essentially without further details on the particular composition or precise properties thereof.
US 2001/0032666 A1 describes a capacitor-type energy storage which may for instance be a lithium-ion battery. Here, semiconductors are mentioned as substrate materials, inter alia.
U.S. Pat. No. 6,906,436 B2 describes a solid state battery in which metal foils, semiconductor materials or plastic films can be used as substrate materials, for example.
U.S. Pat. No. 6,906,436 B2 describes a variety of possibilities for optional substrate materials, for example metals or metal coatings, semiconducting materials or insulators such as sapphire, ceramics, or plastics. Different geometries of the substrate are possible.
In U.S. Pat. No. 7,494,742 B2, metals, semiconductors, silicates, and glass, as well as inorganic or organic polymers are described as substrate materials, inter alia.
U.S. Pat. No. 7,211,351 B2 mentions metals, semiconductors, or insulating materials and combinations thereof as substrates.
US 2008/0001577 A1 mentions semiconductors, metals, or plastic films as substrates.
EP 2434567 A2 mentions, as substrates, electrically conductive materials such as metals, insulating materials such as ceramics or plastics, and semiconducting materials such as, e.g., silicon, and combinations of semiconductors and conductors or more complex structures for adapting the thermal expansion coefficient. These or similar materials are also mentioned in documents US 2008/0032236 A1, U.S. Pat. No. 8,228,023 B2, and US 2010/0104942 A1.
By contrast, US 2010/0104942 A1 describes, as substrate materials that are relevant in practice, only substrates made of metals or metal alloys having a high melting point, and dielectric materials such as high quartz, silicon wafers, aluminum oxide, and the like. This is due to the fact that for producing a cathode from the usually employed lithium cobalt oxide (LCO), a temperature treatment at temperatures of more than 400° C. or even more than 500° C. is necessary in order to obtain a crystal structure that is particularly favorable for storing Li+ ions in this material, so that materials such as polymers or inorganic materials with low softening points cannot be used. However, metals or metal alloys as well as dielectric materials have several shortcomings. For example, dielectric materials are usually brittle and cannot be used in cost-efficient roll-to-roll processes, while metals or metal alloys, on the other hand, tend to oxidize during a high-temperature treatment of the cathode material. In order to circumvent these difficulties, US 2010/0104942 A1 proposes a substrate made of different metals or silicon, wherein the redox potentials of the combined materials are adapted to each other so that controlled oxide formation occurs.
Also widely discussed is how to circumvent the high temperature resistance of the substrate as required in the aforementioned US 2010/0104942 A1, for example. By adapting process conditions, for example, substrates with a temperature resistance of 450° C. or below can be used. However, prerequisites for this are deposition processes in which the substrate is heated and/or the sputtering gas mixture of O2 and Ar is optimized and/or a bias voltage is applied and/or a second sputtering plasma is applied in the vicinity of the substrate. This is discussed, for example, in US 2014/0030449 A1, in Tintignac et al., Journal of Power Sources 245 (2014), 76-82, or else in Ensling, D., Photoelectron spectroscopy examination of the electronic structure of thin lithium cobalt oxide layers, dissertation, Technische Universität Darmstadt, 2006. In general, however, such process engineering adaptations are expensive and, depending on the processing, are hardly implementable in a cost-effective manner, especially if inline coating of wafers is envisaged.
US 2012/0040211 A1 describes, as a substrate, a glass film with a thickness of at most 300 μm and a surface roughness of not more than 100 Å. This low surface roughness is required because the layers of a thin film storage element generally have very low thicknesses. Even small unevenness of the surface may have a critical adverse effect on the functional layers of the thin film storage element and may thus result in failure of the battery as a whole.
The same applies to document WO 2014/062676 A1 which discloses thin film batteries using borosilicate glass or soda-lime glass. No information about thickness variations of the substrate is given there.
Thus, shortcomings of the prior art particularly exist in the area of substrate and/or superstrate materials of electrical storage elements, in particular lithium-based thin film storage elements. This is due to the aggressiveness of the functional materials used for such storage elements. For example, these functional materials necessarily have high redox potentials, especially in the charged state, and are therefore latently instable. If these redox potentials are not reduced in a controlled manner during the use of such a storage element in a discharging process, a reaction of the functional materials with other, even otherwise inert components of the storage element may occur, for example with a substrate on which functional layers have been deposited.
As described in US 2010/0104942 A1, for example, a drawback of lithium as a component of an electrical storage element, in particular of a lithium-based rechargeable battery, is its high mobility, so that under the manufacturing and also the operating conditions of a lithium-based electrical storage system diffusion of the lithium may occur, for example into a substrate or into a superstrate which is in contact with a lithium-containing material. This not only results in depletion of active material in the storage system and thus in a loss in storage capacity, since the diffused lithium is no longer available for the actual charging and discharging reactions, but may also lead to a degradation of the substrate or superstrate material due to undesirable side reactions.